text:
What is a Certification Flight?
After taking a new plane from the manufacturer's factory to its airport base, the plane must undergo a Certification Flight before starting to operate with the airline:

What is a certification flight?
What is the purpose of it?
What are the tasks done on a Certification Flight?
Why doing a Certification Flight if the aircraft has already been tested before?

Related: EASA Aircraft certification and FAA Aircraft Certification. There are multiple certification flights for the process, which can last several years. It's now also common to have multiple demonstration aircraft to shorten this time by splitting the demonstration activities and running them in parallel. Internal tests done by the manufacturer are only for the aircraft development process.

I believe what you are referring to is an acceptance flight, rather than a certification flight. Certification flights (usually there many) are flights which the manufacturer demonstrates to aviation authorities that a new design of an aircraft is safe. However, just because a design has been tested, does not mean a new aircraft is produced exactly to the design. Acceptance flight is a test flight which make sure the aircraft has been manufactured up to standard.
Acceptance flights are also required after major maintenance work or avionics upgrades. Its purpose is to explore the entire flight envelope and ensure emergency / backup systems work as expected. During a normal flight, only a portion of the systems' functions are used. A test flight is a complete evaluation of all conditions, such that malfunctions or deviations will not go unnoticed.
Of course, the pilots will first examine critical systems (e.g. engines, flight controls) on the ground. Depending on the aircraft, the tests to be completed in the air include:

Engine shutdown and restarts
autopilot roll rates, flap extension speed, engine response etc.
Alternate gear extension
Fuel jettison
Over-speed warnings
Stall protection systems
Ground Proximity Warning System (GPWS)
Auxiliary Power Unit (APU) operation at high altitude
Cabin service systems


text:
Why are high-wing aircraft more stable?
A high-wing aircraft is considered to be more stable in a side-slip because of the pendulum effect. How does the pendulum effect increase stability in high wing aircraft?
This question is about high-wing stability, not about comparisons between low and high-wing design also in the question that talks about pros and cons of high and low-wing design, it doesn't really explain the stability from an aerodynamic point of view.

related: http://aviation.stackexchange.com/questions/701/what-are-the-pros-and-cons-of-high-wing-compared-to-low-wing-design

See: 3.3.4. High wing and keel surface

this is not a duplicate of the linked question: it is stated there that the stability is an advantage, but lacks in details explaining where the stability comes from.

I'm with @Federico, the question is related, but this is not a dupe.

@PeterKämpf pendulum effect is also called Keel Effect. I don't know much about this, but the FAA's Pilot's Handbook of Aeronautical Knowledge has a chapter about the aerodynamics of flight in which they explain the keel effect.

I fly both high-wing Cessnas and low-wing Pipers. I find the low-wings to be much more stable overall, and the aircraft owners agree.

@kepler22b: Either the FAA writer means something different, or he/she is clueless. If you want to know what that "keel effect" really is, post a new question and try to cite the section from the Handbook. I'll be happy to discuss it.

Ok. I'll ask a different question

Are they really?
In a high wing aircraft the center of lift is above the center of gravity. This will not increase stability when the aircraft banks, however, as you rightly say, will help once the aircraft is in a sideslip. In order to keep the fuselage at an angle to the airflow, the wing has now to create a sideways lift component, which it does by flying at an angle to the horizontal. This roll angle of the aircraft will shift the center of gravity sideways, so it is not exactly below the center of lift, but offset laterally. This lateral offset seems to produce a rolling moment which works against the roll angle.
Below I sketched the major forces in a sideslip. Note that the aircraft has a sideways speed component which creates a side force on fuselage and tail (green) which needs to be counteracted by the sideways component of lift (blue).

Short Sandringham in sideslip (based on this source)
But still the weight (black) is attacking at the center of gravity, so it will not roll the aircraft, and the lift is in the vertical plane of symmetry, so it, too, will not create a rolling moment. Ergo, we have no stabilizing effect due to the high wing location!
In a sideslip of a high-wing aircraft the windward wing root will see a slightly increased angle of attack while the leeward wing root will experience a reduced angle of attack due to the crossflow around the fuselage. This would indeed create a rolling moment because it will shift the center of lift sideways, out of the plane of symmetry. In a static sideslip the pilot will deflect the ailerons to maintain the roll angle as well as the rudder to maintain the sideslip, shifting the lift vector back into the vertical plane of symmetry. Now any stabilizing effect of the lift asymmetry is on purpose cancelled by the pilot!
However, this is achieved in a low wing airplane by adding dihedral. Therefore, a low wing aircraft can easily be made as stable as a high wing aircraft in a sideslip.
You will notice that the lateral aerodynamic force (green) acts above the center of gravity and also produces a correcting rolling moment. This is dictated by the location of the vertical tail, which contributes little to the location of the center of gravity, but produces a considerable share of the lateral force (reduced in the sideslip case by rudder deflection, however). This rolling moment, though, is almost independent of wing location. In a low wing, the center of gravity will be lower in total, especially when the engines are mounted on the wing, so the rolling moment contribution of the vertical tail is somewhat higher. The effect is small in a sideslip since the rudder deflection means that the fuselage contributes the majority of the side force. And this is not the pendulum effect which you asked about and which does not exist.
In airships, the pendulum effect is real, however: Since buoyancy always acts against gravity, a lateral offset of the heavy gondola will create an uprighting moment, just as it does in a pendulum. When turning, the heavy gondola will be pulled sideways by centrifugal forces, and the airship will roll. Since the turn is commanded with rudder, the rotation will give the rudder a small nose-down moment, which must be compensated by a nose-up elevator command. The pendulum effect will ensure that the gondola is at the lowest point in straight flight.

P.S.: Thanks to the hint of @kepler22b I have now discovered the keel effect page on Wikipedia. It also mentions the pendulum effect and calls both the fuselage contribution to the dihedral effect. Man, if there is ever a competition for the most misleading name of an effect, this would be the winning entry.
A pendulum is a mass mounted below the hinge point, so it will stabilize in the down position. A flying airplane is not hinged, so all motion is around the center of gravity. There simply is no pendulum effect in airplanes.

Good answer, but I have a question. If I look at your picture, and imagine this situation for a low wing, it will result in the same Sideforce, and the same Lift, but then at a lower location. As such both the sideways component of the Lift and the Sideforce will generate a clockwise moment. In the high wing case these cancel eachoter, in the low wing case they will add up. Isn't this a difference between high and low wing?

@ROIMaison: The lateral center of pressure is not where the wing is, but determined by fuselage and vertical tail. In the example it is very close to the wing height (the vertical tail is pointing up, after all), but that will not change with a low wing location. Yes, it does produce a rolling moment, as discussed in this answer

@ROIMaison: The pilot will make sure that the lift vector stays in the vertical plane of symmetry (by deflecting the ailerons), so regardless of wing position there is no rolling moment from the lift vector.

What happens if I choose to use only rudder to accomplish a turn, and not the ailerons? Your answer does not appear to address this aspect of stability.

@JonathanWalters ... because both the question and my answer concern sideslipping, not turning. Feel free to ask, and I will write an answer on that aspect, too. Hint: A lot depends on the $c_{l\beta}$ of the particular design.

Fair enough. The question does specifically mention the slip, but the broader question concerns both slips and skids. I suspect if I asked that question it would be closed as a super of this one.

@JonathanWalters: Just did a quick search on "How turn without aileron". The three results were way off - I guess this question would be genuinely new and should enrich this site. Note that we have a couple of questions concerning the control of aircraft after failures, but if you ask for the physics and unintended consequences of rudder-only turns the question should not be flagged as a duplicate.

So does this mean that a gondola is hinged because it does have a pendulum effect?

@RyanMortensen: Please explain why a gondola has a pendulum effect! And which gondola? The engine gondola of an airship? This one has a pendulum effect, but is not hinged. A Venetian boat? Also not hinged, and it is stabilized by the center of gravity being below the metacenter. No pendulum effect, either.

@PeterKämpf thank you! I was thinking that a pendulum effect was only possible with hinging.  Would an airplane not have a pendulum effect even if you lowered the C.G. to a ridiculous extreme and put it in a much steeper bank than is shown in your diagram?

@RyanMortensen Not as long as the turn is coordinated. Then the link to the weight will point directly against the lift force, so no moment is created. Once you put the weight to a lateral offset, then yes, then you get a moment. This is used in hang gliders for weight-shifting control. Hang gliders should be closest to the configuration you propose, and the pilot will be the weight.

@RyanMortensen: The supposed pendulum effect demands that the whole aircraft acts like a pendulum and is hinged at the center of the wing. This is clearly a logical fallacy. You seem to think that it requires a weight which can pivot around a hinge, a real pendulum, as part of the aircraft. That is not what the believers in the pendulum effect myth think.

@PeterKämpf it isn't what they think?  If the pendulum effect myth is not about a hinged point, then what is it that people are thinking is happening?  P.S. I think I get it now.  Are they looking at the C.G as the hinge and the lift as the "weight" being sort of an up-side down pendulum?

@RyanMortensen: It's even worse: As I understand it, they think the wing root is the hinge. The weight is the center of gravity, and if it is below the wing, this is supposed to stabilize the aircraft and pull it out of a roll angle.

@PeterKämpf perhaps another way of trying to explain this (or getting it to click for some people) is  by saying that the C.G. is a hinge point and that the C.G. is also the pendulum-weight, but they are co-located, therefore the arm is zero and the moment is therefore, also zero no matter what the magnitude of weight is or where the C.G. is located.

@RyanMortensen: Yes, you're right. But such memes are hard to kill. Just think of the "tip vortices cause induced drag" meme. Like saying that wet streets cause rain. The most correct explanation would be: The free flying aircraft is not hinged at all, and all rotations occur around the c.g. Then your co-location point kicks in.

@ROIMaison if you like take a look at my answer which tries to explain it simply

@PeterKämpf Please consider "pendulum effect" https://aviation.stackexchange.com/questions/53437/does-pendulum-effect-in-aircraft-actually-exist-or-is-it-a-misunderstanding-of

To add another voice to the fray, Peter is absolutely correct. The pendulum effect is fictitious. In fact, "weight" is actually fictitious as well, as anyone with a passing familiarity with Einstein's General theory will tell you. The force of "Gravity" is only a fictitious abstraction due to the fact that we are doing our analysis in an accelerated frame of reference (the earth). Exactly the same as if we were examining the flight of an aircraft in a giant box filled with air in outer space, that was accelerating upwards at 1 "G", from a giant rocket on the bottom.

To have a simple sense about it, we can consider when an high wing aircraft is side slip to the right as shown in figure, there is a high pressure region near the fuselage below the right wing (windward wing, the direction from which the wind is coming) and a low pressure region on the other side.

This will increase the lift of right wing and reduce lift of left wing, so a moment will be produced which tries to roll back the aircraft towards normal position.

Reminds me of this question, where the air pockets would be created by the doors of a small airplane.

You are right, a high wing contributes some dihedral effect. But that is not directional  stability by itself - add more dihedral on low wing airplanes and both come out the same.

Love the diagrams.  Does a parachute have no pendulum stability either?
Try scratch building gliders.  What you can learn from paper, glue, and balsa is amazing!
Yes, a high wing has pendulum stability, as do HANG GLIDERS!  As a matter of fact, the hang glider adjusting to (pendulum) weight shift ELIMINATES the need for control surfaces!
Pendulum effect can not be described aerodynamicly because it is NOT an aerodynamic force! This is a hard learned lesson for people who try to "trim out" a CG that is too far back.
In scratch building gliders one learns at higher speeds, aerodynamic forces rule, but as one slows down, weight distribution factors in more and more.
An aircraft, IN STRAIGHT AND LEVEL FLIGHT, the CG will always try to be directly under CL.
On the flying boat, breaking the lift components down to vertical and horizontal may help clarify the point.
For "pendulum effect" to work, weight underneath the
center of lift (such as landing gear) is displaced from being directly under the center of lift. As the plane rolls, the center of lift in relation to CG can
change.  Easier to see in a hang glider or airship, but still present in a high wing aircraft.

Hang gliders only shift their wing relative to their center of gravity. See here for more.

@Peter Kampf I believe this discussion has been resolved for turning and non turning cases.  In the non turning case ANY displacement of CG from directly under Clift will result in a rolling moment to restore the alignment of lift and weight in the gravitational field (defining "lift" as all upward forces including buoyancy). Turning cases are indeed another story worthy of close examination and thought.

@Peter Kampf after studying many of the beautiful mid wing glider designs of the 1930's (which also included drag reducing fairings) I can see how a whole generation of people grew up on center of gravity theory, indeed that was a GOOD place to put the wing for light, responsive roll control also needed in giant airliners.  But, alas, the wing, or the ballon, or the parachute, or the hang glider, or a string, holds it up against gravity.  That is reality.

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What are these "lines" on the A319 flaps, are they damaged?
I've noticed a grey line on the flaps of the A319:

What is that?
Source

duct tape holding the flap together ;-)

You are seeing the steel reinforcing sheet metal at the line where the spoilers rest on the flaps when both are retracted.  It is there as a bearing surface since there is some inevitable contact between the spoilers and the top surface of the flap when they are both retracted during flight.

From a maintenance point of view, do you known how long they are used before the sheet are changed?

I don't know if they have a regular maintenance schedule or wear limit on that sheet metal.

The technical name is Rubbing Strips.

If you painted those strips the paint would rub off unevenly and look like damage.

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text:
Who manages International Airspace Traffic?
For example, if a plane is flying from Amsterdam to Miami and is in the middle of the ocean, who manages that traffic? The ARTCC? Or the airplane just flies without communicating for some hours until it reaches an ARTCC area?

Global ATC in general
In the civil aviation domain, international rules are proposed by ICAO. The foundations of these laws have been agreed in 1944 under the Chicago Convention, in particular its Annex 2, Rules of the Air. Nearly all countries are signatories of this convention.
ICAO global airspace structure is divided into large Flight Information Regions (FIR -- their upper layer being conventionally named UIR). A FIR over a domestic airspace is under the responsibility of the country below. A FIR over the international airspace (e.g. airspace over international waters) is under the responsibility of a single ICAO member state which has received a delegation from ICAO.
Within a FIR, enroute services are provided by Area Control Centers or ACC (ARTCC is the FAA local name for ICAO ACC). The first services available (and guaranteed) are information and alerting. Control services like separation can be provided, depending on the FIR.
(That's unfortunately a lot of acronyms, and more are coming soon... I give them only for precision in case someone wants to dig further into details, but you don't need to remember them to understand the system.)
Specificity of Oceanic Control
When a FIR provides control services over an ocean area, such area is named Oceanic Control Area or OCA, and the corresponding ACC are named Oceanic Area Control Centers (OAC). OAC and ACC differ by the procedures they can use to provide their services: ACC are able to track aircraft using reliable radars, while OAC mostly work with voice/data position reports from crews over unreliable long distance communications (that's less and less true with modern means like space-based ADS-B and ACARS/CPDLC/DCPC (see Guidance Material for ATS Data Link Services in NAT Airspace here).
North Atlantic case
All flights which want to maximize (eastbound) or minimize (westbound) the effects of high altitude jet streams of the day over the Atlantic Ocean must use the crowded North Atlantic Organized Track System (NAT-OTS, not to be confused with NATS Holdings, the UK ANSP) when flying at or above FL55. They must obtain an oceanic clearance from the first OAC before reaching their entry point into the NAT-OTS. So to answer directly your question:

The crew using the track system will be in contact with the OCA/OAC controllers during the flight between domestic areas. However to bear with possible unreliability of HF radio transmissions, this contact doesn't have to be permanent. Instead it may occur only at intermediate reporting points spaced by 10° longitude.

They will also be in contact with FIR/ACC controllers prior to entering and after leaving the OCA/OAC.

Transition areas (OTA) exist to bridge between ACC and OAC: See areas ending in "OTA" in the Shanwick FIR/OCA, like BOTA for Brest OTA, bridging Shanwick OTA (UK) with Brest FIR/UIR (domestic France). In OTA, traffic is handled by the regular domestic FIR (actually its UIR, given the altitude flown at this stage) rather than the OCA.

North Atlantic ocean is covered by 6 OCA managed by different ICAO member States: UK (Shanwick), Iceland (Reykjavik), Canada (Gander), Norway (Bodø), USA (NY West) and Portugal (Santa Maria).

North Atlantic OCAs (source ICAO, now removed from the site)
A common route for the flight you mention will cross Shanwick and Gander OCA.

(source FlightAware)
FIRs and their boundaries worldwide can be displayed with this ICAO GIS viewer (be patient at the first use, background resources take time to load):

(source: ICAO FIR viewer)


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Why is this 777 with doors and windows covered?

It's in the desert somewhere and it has its windows and landing gear and doors covered. Why is it that? Of course, it's not in service anymore...
Source

On a random note: This airplane actually looks like a balloon under the lighting conditions and the coverings.

The legend of this image on the original site is quite informative: "EI-FGT (cn 28530/390) Ex Singapore Airlines 9V-SQI, in deep storage configuration for almost a year now (arrived in VCV 2/23/15), and so far seems to have an uncertain future. VCV storage.

That's in Marana, Az in the United States and it's been decommissioned.

@Citizen: Welcome! You are right about an aircraft boneyard location, however the photographer indicates the photo was shot in Victorville Southern California Logistics airport (former George AFB - VCV). More on VCV.

Oh nice.  I missed that.  Thanks.  I'm in Az and go by the Marana boneyard occasionally. As well the former afb in Tucson.  Looked just like Marana to me. Ty for clarifying.

The aircraft is in storage. The Boeing 777-212(ER) seems to operated by Singapore Airlines under lease from Pembroke before being withdrawn from use in Dec 2014. After that, the aircraft is under storage at Southern California Logistics Airport.
The windows are covered in aluminium foil for storage; however, the engines appear to be still there (which will usually be removed), which indicates that the storage was planned for only a short time.

Currently that airplane is in storage per planespotters.net. The reason why it is stored in the desert is because the environment would not cause some structural damage to the airframe. All those doors and windows are covered to keep animals, dust, and even sunlight out so that they would not damage the airframe either.
You can read more about storing aircraft in the desert here and here.

The aircraft was previously operated by Singapore Airlines but has been returned to Pembroke Leasing. It is probably in storage because it is not economical to sell or lease at the present time.
Here are some possible reasons why it may be uneconomical to sell or lease:
1) excessive supply of aircraft for market conditions
2) cabin or seating configuration
3) excessive hours on the aircraft, engines or components
4) age of the aircraft (14 years)


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Why do some airports put 2 jetbridges on one aircraft?
At a gate in Denver Int'l there is a United plane with 2 jetbridges.

Why do some airports put 2 jetbridges on a plane? Is it necessary?

It is actually very common to put multiple jetbridges on wide-bodies. The A380 uses 3.

Some actually have three for some planes

Why do you assume that they have to?

Note that for instance EasyJet in Prague (PRG) uses a jet bridge plus it makes the passengers seated at the back of the place to go down on the tarmac and up by standard stairs. I believe this significantly reduces the boarding time whence the airport tax, which is quite large in PRG.

@yo' is the tax really time-dependent? Normally easyjet etc. want to minimise the turnaround time for their own operational reasons rather than anything external.

@ChrisH Honestly, I don't know,it was just an assumption. Of course, the operational costs could be an argument as well.

Look at the size of that aircraft and guess why! Who wants to keep the gate busy when they can utilize two bridges to transfer people faster than one on big aircraft

Confusing question ... They hardly "HAVE TO" ..... it's just much, much better.  You may as well ask "why do some airports 'have to' use bridges rather than just a bus and steps.

@kepler22b I don't understand. The title used to be "why do they do it?" and the question used to be "why do they have to do it?" You get a bunch of comments saying that they don't have to do it so the real question is just "why do they do it?" But then you edit the title so it includes the incorrect "have to", rather than editing the body to remove it. IDGI.

Because they can.

So that economy class passengers can also see at least one of the higher classes (when they exit) and wish they booked that!  :-/ Trust me you sure will wish that and that someday can mean more $ for the airline

Source: Original picture was taken from this very question
Joking aside I have seen it happen on many airlines. The people on left Aisle go out from the first one and people on the right aisle take the longer route to exit from the gate that's closer to cockpit.
Primary reason must be to transfer people faster to make the jet available for crew as soon as possible specially after a landing when the jet has to fly again in an hour.

This answer is inaccurate or, at least, incomplete. I've never seen dual jet bridges used in the way described here. On all occasions that I've boarded a plane that had two jetbridges attached, all economy passengers went through the rear jetbridge, presumably to avoid disturbing the people in the fancy seats by having half a planeload of people trog past them.

So if you have never seen that happen that means that is inaccurate? I have recently seen it happen twice, once at KUL and once at BKK. And those fancy seats people offload before not so fancy ones do. Both on TG.

@DavidRicherby write your own answer if you think this one is inaccurate.

@HankyPanky I said "inaccurate or, at least, incomplete". There are many occasions on which dual jetbridges are used but are not used in the way you describe.

@kepler22b There are plenty of answers that already cover this.

But if you are really annoyed by this answer just make your own answer correcting this one. Because first you said that the title was wrong, then that the answer is inaccurate, what else comes next?

It is not strictly necessary. But it is done in order to save time.

More points of passenger loading leads to faster loading, potentially reducing delays, airport charges, etc.

In some cases, there is a separate jet bridge for first/business class.

For large aircraft like A380 etc, is quite common to use multiple jet bridges, as use of a single one may lead to quite long loading time.

Also, some planes have more than one floor for passengers and they might not want everyone to go through the narrow stairs.

@StephanBranczyk exactly; not a big deal for 747 (though you don't want your upper floor/class passengers to have to climb) with it's handful of seats on the upper deck, but in the 380...

The reason they use 2 or sometimes even 3 (A380) for wide-body aircraft (I have seen 2 on a B757, but that is rare) is because the more entrances to facilitate faster boarding and unloading passengers from the airplane. It saves much more time than loading the plane from one jetbridge and in the case of the A380, there are 2 full wide-body decks to fill. Passengers also like getting on and getting off the plane quicker as well.
Interesting that there is this similar question on travel.se

Believe it or not, a decade ago, Southwest experimented with using two jet bridges for boarding 737's at the Albany NY airport.  It really did make boarding faster but I haven't seen done recently.

In addition to Aeroalias' answer, some airports adopt a Multi Aircraft(Apron) Ramp System.
The gate may be used by multiple small or a single large aircraft. It allows airport planners to make their gates more flexible and efficient. In such a case two or more jet bridges are needed for a single gate.
Some of the busiest airports including Beijing Capital, London Heathrow and New York JFK have already adopted such a system.
More details can see this article.

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text:
What is the difference between a Sub-Charter Flight and a Codeshare Flight?
What are the main differences of this two types of flights?

I don't know if it still applies, but back in the 1990s a codeshare flight was a regularly scheduled flight of a single aircraft identified by the flight numbers of two different carriers, both of which sold tickets for the flight. A sub-service flight, which I'm guessing that sub-charter refers to, was when airline A called airline B, often on short notice, and said something like, we don't have an airplane for our flight 1234 Montreal to JFK. Can you cover it for us. That's often expensive for airline A because of empty positioning legs.

Sub charter is not used nowadays as a word expression. It's now called wet lease or ACMI. An ACMI specialist airline does not fly its own regular schedule, but usually flies for other airlines on shorter/longer notices (couple of flights or whole schedule). When WOW air started in Iceland, its schedule was flown by a Lithuanian ACMI operator. HiFly flies for Condor its service from Munich this winter, because Condor's 767s are undergoing heavy maintenance. Thomas Cook of UK is a permanent wet leaser to Condor (they are in the same holding). They provide aircraft and pilot crew, but Condor does fuel, maintenance and cabin crew. In the HiFly example above, HiFly provides full ACMI (aircraft crew maintenance insurance) for Condor.


text:
Are these hangars?
I found an AFB in Germany that has some sort of hangars. Look:

And here there are more but in other AFB:

Are they hangars? If so, why are they covered with grass?

old WWII camouflage maybe?

https://en.wikipedia.org/wiki/Hardened_aircraft_shelter, here's an image

Another question about dispersal.

They are hardened aircraft shelters (HAS) designed to protect aircraft from air attack.
https://en.wikipedia.org/wiki/Hardened_aircraft_shelter
In the first picture, they are also on a "dispersal" to spread them out and make them harder to target.

Dispersal is less about making aircraft or other ground assets harder to target and more about reducing the amount of damage that can be done by any single bomb. It turns one big target (a tight group of parked aircraft) into lots of small targets.


text:
What does this vehicle transport?
I've seen on many airports a lot of vehicles transporting some "brown boxes".

Any idea of what is this?
 (I don't ask what's exactly inside this box, just in general)
Source: 123rf.com

What that may be?  Baggage? : in case that boxes contain baggage, why don't they put it as loose baggage or in ULD's. May that be other type of cargo?

Where was this picture taken and when?

Sao Paulo Guarulhos 9/21/15

Do you remember in which company they were loading in?

It was a SWISS A340

Brazil also exports organic cocoa to Switzerland, that might be what you are asking for.

Lots of cargo is packed in cardboard before being packed on a pallet.

Why is it that?

@kepler22b: Are you serious? What do you pack things in to move them from your attic to your cellar? Discarded refrigerators?

Well, some people said that those boxes may cointain baggage

Also, editing in the source of your images is generally considered good form, though the Getty Image contains obvious copyright info. It might be wise to see if you can find a non-copyright image to use in its place (unless, of course, you have permission to use the image).

Those are just cargo AND passenger luggages. Yes ! Cardboard boxes aren't that strong (you don't want to put a 50-60lbs luggage on top of), but they have a nice tendency to fit very well when stacked one on top the other. So, good practice is to group and tie/secure them on palettes for ease of loading and space management. Being passenger boxed goods, mail, non passenger cargo, special cargo like dozens of microwaves (...) if it's a cardboard box, put them together with other cardboard boxes. As simple as that. There is nothing mistical or disturbing about, standard/legal/efficient practice.

Your question is rather broad and somewhat unclear, but what you have is a picture of a baggage tug and cart.
The carts are loaded with passenger baggage or cargo, then strings of them are pulled to the aircraft by the tug before being loaded on to the plane.
Based on the additional pictures, I think what you're after would be called palletized cargo. It's stuff, put in boxes, placed on pallets, wrapped with stretch wrap, with a net thrown over the top and hooked down to keep it from sliding off the pallet.
It is most likely not passenger baggage. That's either loaded one piece at a time by hand, or in a ULD.
If you're asking what's in those particular boxes, you'd have to zoom in quite close to find a label on a box to even have a chance of figuring it out. Just about anything can be and is shipped in a brown cardboard box, from anywhere to anywhere in the world. This question is far too broad and off topic for Aviation.SE, and probably any SE.

humorless downvoter seems to have struck here, too. :(  Considering the question was nearly closed when I originally answered, I don't think it was that bad...

It's too small a change for me to suggest an edit, but you want palletize (to place on pallets) in place of palatalized (a phonetic term).

@JeffreyBosboom thank you sir. That's what I get for relying on the red, squiggly underline, instead of thinking...

Those are cargo and baggages. Yes passenger flights do transport cargo other than baggages.

shipping batteries could be a possibility.

Not on a passenger flight, it couldn't. But it could be millions of other things.

humorless downvoter has no humor


text:
Why are the 747-8 engines placed differently than on the older versions?
On the 747-200 the engines were tilted up to "meet the air at a streamline angle":

Source: Wikimedia Commons

Wikimedia Commons

Source: RcGroups.com
But in the 747-8, the engines are not tilted up:

Source: Airways News
Does anybody know why the engines are not tilted up anymore?
Thanks!

The -400 has no tilt too: http://www.avionale.com/2010/04/lufthansa-boeing-b747-400-wallpaper-706.html/

tilted up to "meet the air at a streamline angle" Where did you quote this from?

@TomMcW From this question: http://aviation.stackexchange.com/questions/1543/why-do-dc-9-and-md-82-have-engines-that-are-slightly-angled-upwards

That question is relating to DC-9/MD-80 which have rear-mounted engines. The angle on those is related to them being behind the wing. I looked at a bunch of pics of 747-2 and they don't appear to be tilted upward. Are you sure it's not just the angle of your photo? Usually wing-mounted intakes are slanted slightly downward if anything like in the B737

@TomMcW I've put some other pics to show that the engines are tilted up :)

Cool. I'm hoping somebody answers this. I always thought you wanted thrust vector to be pretty much parallel to the chord and the intake slightly downward to maximize efficiency during the higher AoA during take off and climb. I could swear@PeterKämpf talked about this before in an answer but I can't find it

@TomMcW: Yes, I did, but that question asked why the intake plane is tilted downward. This is to balance the flow conditions between cruise and take-off, but the reason for the unusual tilt of the 747-200 engines is unknown to me. I guess this is an artifact of wing twist - when loaded, the swept wing will reduce its local incidence.

On the 747-8 the engines are heavier and mounted farther ahead of the wing. This results in a torsion moment which reduces the incidence of the wing. As a result, even the unloaded wing will have a lower incidence than that of the older 747-200.

Boeing 747-346 of Transaero (picture source). Note that the engines are no longer tilted up (and consequently the wing tip is tilted leading edge down). The lift force twists the wing such that the incidence in flight is lower than when unloaded and on the ground, especially when the high-lift devices are extended.
Also please make sure to read @Lilibete's answer. I don't know who thought it appropriate to downvote it - it contains another reason for the change in inlet angles, namely the difference in avoidance strategies for foreign object damage.

@ymb1: Thank you for the hint, and next time you may very well do the change directly. It is a clear improvement.

Peter's answer explains one aspect of the appearance of the engine angle being the way it is between the -200 engine angle relative to the angle of attack on the ground versus in the air and between the -200 and the -8.
There are also the considerations of FOD (foreign object debris) entering into the engine and the account for accessories off of the engine that are concealed by the cowlings. FOD is handled very different today (using bleed air screens) than when the -200 was designed when the angle on the engine's inlet was used. As far as the accessories that are concealed under the cowlings, they are also vastly different.
Another thing that drives the shape and size of the fan cowls is the bypass ratio. For the -8, the bypass ratio at takeoff is 9.0/1, so 90% of the air going through the fan goes into the bypass section. For the -200, the bypass ratio is 5.72/1, so roughly 83% of the air is bypassed. While the doesn't seem like a large difference, the geometries are significantly different. Those differences mean that the risk of FOD going through the fan versus the core are different and yield different inlet angles.
Other factors that make the appearance of the overall angle being different between the two versions are going to be side slips inlet designs and just the camera angles taking the pictures. The overall angles of the engine itself may not actually be that different.

David, Peter's answer explauns why the angle appears to change due to the aerodynamic loading and unloading of the wings. We could talk appearance in this thread, but we won't get the answer of an actual angle because that would hold trade secret information that is very specific to performance.

Your explanation of the bypass ratio is off. 100% of the air passes through the fan always. then 90% goes around the compressor and turbine stages to produce the thrust and the remaining ten goes through these stages to produce the power required to run the fan. Note this is for the 9:1 bypass ratio.


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How to know if an aircraft is climbing or descending?
As a ground observer, how can you tell if an aircraft is climbing or descending?

Do you have an ADS-B receiver? :)

Know the flight patterns of your area airports. Also, some aerobatics pilots and student pilots doing training are really easy to just listen to tell (Engine goes to idle: Descending. Engine roaring: Ascending).

@SMSvonderTann That will often be true, especially in cases of large power changes. However, in a recip with a fixed-pitch prop, a climb with no change in power (and attendant drop in airspeed) will result in a decrease in engine RPM and may appear to the ground observer to be a power reduction. By the same token, a descent (and attendant increase in airspeed) will result in an engine RPM increase and the appearance of a power increase.

It it is an airliner, get yourself the [flightradar app]/(http://www.flightradar24.com/apps)! The free version has a point feature that allows you to know climb rate, speed, model type, tail number, where it goes and where it comes from!

@SMSvonderTann Note the pitch change is subject to the speed of sound.  If the plane isn't right next to you there will be a noticeable lag between what you see the plane do and what you hear.

Listen. Climbing aircraft need more thrust, so the noise they produce is related to their vertical speed. Since most excess performance is available at low speed, a slowly flying aircraft producing a lot of noise will most likely be climbing. Once you gain a little experience, the pitch of the engine and propeller sound of piston-engined aircraft will be a very good indicator of their thrust setting.
This doesn't work for gliders. Here you need to wait to be sure if they are climbing or descending. Only when you see that the glider flies with speed brakes extended you can be sure instantly that it is descending.

I would argue the Doppler effect makes observing changes in pitch or volume unreliable when considering climbing or descending.

@RonBeyer: With the slow speed of propeller aircraft the Doppler effect is weak, especially if the aircraft is flying overhead. And this is the position where vertical speed is especially hard to judge from the ground.

Even so, its still difficult to determine relative power for pitch (climbing/descending/cruising) unless you hear the change in power as well. I'd say that yes, you can use it to observe rapid changes, but living near an airport usually an aircraft starts its descent long before you can hear it.

@RonBeyer: Don't forget the speed of sound: The change in engine pitch reaches your ear with a delay of one second for every 1000 feet of distance. When you see that the airplane begins to descend, you will hear the change in pitch several seconds later. If you know the type of aircraft well, you are very well able to judge its power setting by listening.

The listening option will often be true, but recips with fixed pitch props will not necessarily follow this rule of thumb, especially from the perspective of the uninitiated. The frequency of prop noise, a function of RPM whether driving or driven, will have no correlation to vertical speed.


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Fighter jet with two rudders, the deflection angle it is not identical. One have more angle of deflection. Why?
If an aircraft have two vertical tails then pilot push in the  rudder  pedal the rudders will not be deployed at the same angle.It is used throughout the all envelope of flight or is it possible only for low speed? Why and which ones  is  more deflected?

You know for a fact that all fighters with twin tails deflect the rudders at different angles?  Because I think you may be making a faulty assumption.  But, if you have a specific and verifiable example, please share details.

Yes, I seen that. The out of the turning rudder is more deflected than the other one.

It is used throughout the all envelope of flight or is it possible only for low speed?

Do you see that in which aircraft in which situation? You ask as if it was true of all of them, but it is unlikely the same in all types. Do you mean F-14? MiG-29? F/A-18? De Havilland Sea Vixen? Su-27? De Havilland Venom? F-22? F-35? Very different designs, with very different requirements, made by different companies…

The aerodynamics is the same for all of those rudders. If they are spaced enough then it will be deflected at the different angles.

I wouldn't take that picture as evidence of different rudder deflections: the picture isn't taken from exactly head on. The nose looks like it skews to our left, and whereas external fuel tank on our right is pointed straight forward, the one on our left looks pointed outward. I highly doubt that the nose is asymmetrical and the left fuel tank was mounted at an angle; we're just seeing the plane from a bit of an angle. I think that may account for the rudder's perceived deflection, too.

of course even if they're not meant to, a malfunction can cause this to happen. And my guess is it'd cause the pilot some headaches before he figured out how to compensate for the asymetric rudder deflection just as a faulty rudder would cause trouble with an aircraft having only one.

There's others in flight

Somewhat related

This feature may be quite different for parallel rudders (e.g F15) and V-shaped one (e.g. F22, or in the opposite direction the A12), for fly-by-wire aircraft (in which you can virtually deflect any surface in any direction independently) and for conventional transmission (e.g. P38) (in which there are physical links limiting independent deflection of both rudders)

True, the F22 doing the pedal turn definitely not uses the rudders in their symetrical way, or F117 in pitch up.

The jet in the top picture is turning slightly left using nose wheel steering, which is generally the same control as the rudder. I'm not aware of any fighter jet that has a tiller. Therefore the computer has commanded the rudders to deflect left. This is likely combined with a "toe in" (a characteristic of some fighters that aids in stability) of the rudders so it looks like only one has moved.
In the bottom picture, the flight control computer has decided that the best way to maintain the pilot's commanded attitude is to asymmetrically deflect the rudder. Flight control computers move surfaces in fly-by-wire systems, not pilots. Pilots just ask the computer if it is okay to do that. The computer decides if it is okay and how. It will use a combo of all the available surfaces to achieve the desired attitude within the programmed flight envelope and it doesn't have to do it symmetrically.

The toe-in not "tow in " it is for pitching up or rotate .The toe out it is used  for airbrake some fighters (F22 ,Superhornet ,F35).


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Which way does the F-16 cannon rotate, and why in that direction?
The F16 cannon is turning when fire the bullets. What is the direction when looking from the front of the aircraft towards behind?

https://www.f-16.net/f-16_armament_article5.html

"Why is it not in the other direction?"  Well, you gotta pick one or the other, right?!  ;)

If it was turning the other way, it would be sucking in bullets.

Sorry, I only know the direction when looking from the rear of the aircraft towards forward.

On a more serious note, the direction of rotation of the barrell assembly is most probably chosen such, that it will enhance the stability os the ammunition trajectory,  possibly enhancing the efficiency of the weapon in the same time (I'm guessing very little, but it is possible though). I do not know which way this would be, the same or opposite to the rotation of the ammunition itself.

The article Ron linked says the rotation is ccv viewed from behind the gun.

Probably in the same direction as the rifling in the barrels. Why design the weapon to reduce the stabilizing effect of the rifling (by rotating the barrels opposite the rifling ), when you could just as easily increase it by rotating the barrels in the same direction? and the rpm of the M-61 cannon at high rate is 1000 rpm, which is significant. The M61 barrels uses progressive right hand parabolic 10 inch twist with a muzzle velocity of 3200+ fps.  Without rotating the barrels,  that equates to the bullet leaving the barrel at about 4000 rpm. Reducing that by 1000 rpm would be significant.

Charles Bretana, very good explanation

As it turns out per WPNSguy's answer, and Charles' comment, the barrell assembly and the bullet are "counter rotating". WPNSguy's reasoning about this is plausible. There is in fact, such a thing as "overstabilized bullets" where the bullet spins too fast, and will not conform to gradual change in speed and trajectory. This may have been taken into account in gun rifling design, probably has been.

I need to revise my comment... Actually, although this seems like a very large number, leaving the barrel, the bullet is spinning at about 240,000 rpm.  The 4000 value I incorrectly mentioned above would actually be 4000 revolutions per second, since it is based on muzzle velocity of 3200 fps. So adding or reducing the rpm by only 1000 would not be that significant. It still seems like it would make sense to increase rpm rather than reducing it.

As said in the f-16.net link from Ron Beyer (and many personal years working on them by me), CCW as sitting in the seat. So CW looking from front to back.
Why?
You'd have to ask the designers. But one possibility might be that the rotation imparts a little bit of sideways momentum as the round leaves the barrel end. You'd want that sideways bit to be away from the aircraft body, not towards it.
Pure conjecture on my part, though.

hmmm, so you CAN curve the bullet...


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The instrument that show the attitude in the F-16 cockpit in a dive glide slope but nose up attitude what will show?
An F-16 Fighting Falcon is nose up at 25 degrees angle of attack (AoA) and unable to keep the level flight path  but it is in a $600$ feet per minute dive slope. Does the attitude direction indicator (ADI) show more of the sky or more of the ground?

Angle of attack is not a variable in the pitch attitude of the airplane.  You can be at any AoA at any pitch attitude.   What is the power setting?

The power setting is on the left side of the curve ,but I'm interested only the attitude showed on ADi ,regardless of the /climb /dive (here)velocity.

600 FPM is 3 degrees or so flightpath below the horizon at normal approach speeds; it would be perhaps 1 degree below the horizon at 250 knots. So if your flightpath is -1 to -3 and your AoA is +25, your pitch can be estimated, as long as one assumes thing like wings level and some sort of reasonable airspeed. (If assumptions can include unreasonably slow forward speeds, ie a VTOL hanging in the air with barely any forward or vertical speed, then everything is "possible", if pretty wildly hypothetical.)

"An F-16...  is nose up at 25 degrees angle of attack (AoA)" -- this sentence is confusing.  The implication is that "25 degrees" is meant to specify exactly how "nose up" the plane is.  It sounds like you meant to say the pitch attitude is 25 degrees.  Did you?  Or did you mean to say something like "The nose is above the horizon at some unspecified angle, and the angle of attack is 25 degrees" --?  Pitch attitude and angle of attack are two completely different things.  Or by "is nose up" did you not actually mean that a/c was in a positive (nose-up) pitch attitude? Please clarify.

(As long the a/c really "is nose up" relative to the horizon, the answer seems obvious.)  On the other hand if the question is only meant to specify that the a/c is nose-up relative to the flight path, then you really need to say that explicitly, because that's not what we normally mean by "nose up".   As it stands now the question is confusing.

Please do not make pointless edits to closed questions, as these serve no good purpose but "bump" the question to the top of recent activity.

If the nose of the airplane is above the horizon the ADI will show more sky than ground.
Rate of descent, AoA, and power setting are all irrelevant.  The ADI displays only attitude with respect to the earth's horizon.

Adding to this excellent answer, (totally correct without any irrelevant additional details), in the HUD, which also displays aircraft attitude as the HSI does, among the other info displayed is a flight path marker, which is positioned over the spot representing the aircrafts current wind- corrected flight path or velocity vector. The difference or displacement between the two represents the aircrafts AOA, sideslip angle, and any wind correction.

true , but I want to isolate the ADi .

@GeorgeGeo what exactly are you trying to isolate? Michael Hall’s answer is correct. The pitch attitude of the aircraft (what is shown on the ADI) is what matters, not the AOA or velocity vector.

@Timbo, I think George was referring to Charles' comment.  (i.e. he isn't interested in  extra information about the HUD, sideslip, velocity vector, etc...)

The only reason (albeit a minor one) to add these extraneous details, would be to educate the op about what these other variables (like AOA, sideslip angle, etc. etc. ), are, how they relate to aircraft pitch attitude, which is displayed on the ADI, so that the answer to his question might make more sense to him. As his question, as worded, might betray a fundamental confusion about the relationship of these concepts.

Charles, I always welcome feedback, but this is the second time recently that you have suggested a lot of what you admit are extraneous details to an answer of mine.  You are super smart and experienced, but please answer yourself if you truly think the information is useful.  I'm not saying it isn't, just that while I might sometimes expound significantly, other times I choose to simply answer what was asked.

@Michael, I am sorry you are so easily affected by my contributions to this forum. Indeed, when I contribute to a thread here where you are involved, to eliminate potential misunderstanding, I have made a conscious effort to include comments that acknowledge and praise your contribution to the issue at hand. However, I do not participate or contribute here to stroke my ego, or anyone else's, or to accumulate reputation points. I participate to contribute, when I can, to a more thorough and complete understanding of aviation and aeronautical concepts within the general aviation community.

@Michael, I deliberately added these extra details as a comment, rather than as a separate answer, because, indeed, your answer was totally correct, and directly answered the op's question. It does not need any additional details. What I added in a comment would on no way be a complete answer to his question. It would be incomplete and confusing as a separate answer.  However, in future, when I comment on one of your posts, I will word my comments to make it more explicit that I am not implying any fault or criticism of your answer.

@CharlesBretana, there's no need to apologize.  I am not easily affected. (much less offended...)  Don't treat me with kid gloves.  We are both here for the very same reasons, and have also tried to thank and praise you.  It's just an observation.  Simple questions sometimes deserve simple answers, that's all.  I try to match my level of effort with the OPs effort and apparent knowledge.

I'm not complaining... AT ALL.  But if I were to complain it would only be that we end up in a long conversation like this...  But I get it too, there are others besides the OP who might learn something.

I'm glad to hear you say that. No worries with me then.  I am probably overly concerned about how my pedantic nature can at times be misread by others.


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How challenging is it to land a plane on a tabletop runway?
Yesterday(August 7 2020), an Air India Express plane skidded of the runway(Dubai - Kozhikode, IN) and fell almost 30 feet below from the tabletop runway and broke into two pieces. It was also heavily raining and the visibility was low. Unfortunately, 17 people died including the pilot and co-pilot. The pilot was well experienced and had worked in the Indian Air Force. For more info: https://www.msn.com/en-in/news/other/air-india-flight-from-dubai-crashlands-in-kozhikode-both-pilots-among-17-killed/ar-BB17I15z
The runway as I said earlier is a tabletop runway, I heard that these runways create some sort of optical illusion, thus it is challenging to land a plane.

From the limited info in the linked article, it would seem as though a very wet runway that limited braking might have been more of a factor.  In my (limited to single-engine GA, but including airports like this one: https://en.wikipedia.org/wiki/Sedona_Airport ) experience, the only challenge is accounting for possible downdrafts at the approach end.

There are a couple of optical illusions that can come into play on tabletop runways:

Because the runway sits higher than the surrounding terrain, depth perception can be thrown off and the runway may seem like it should be lower than it actually is.

Due to the steep drop off just past the end of the runway, it can be difficult to judge the remaining length as it seems to stretch on into the landscape beyond, almost like an infinity pool.

Beyond this, tabletop runways in general tend to punish overruns more than traditional runways because there can be very limited space before the large verticle drop. Precise, well-configured approaches are vital, and proper care must be taken to ensure that the runway provides plenty of length for the landing roll of the aircraft (considering speed, flaps setting, weight, runway condition, etc.)
To simplify: There are plenty of runways that are trickier to shoot, but might not have such a severe penalty for getting it wrong.


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Clarification on autogyro thrust formula
I'm slightly confused regarding the formula quoted here:
How to calculate the amount of thrust of a autogyro during auto rotation?
When they say thrust is equal to the weight.  Do they mean use earth weight directly i.e. 50Kg or am I supposed to calculate thrust as weight force, i.e. F = m * g?
If its the latter can I just use summary of component forces to determine vertical thrust, resultant from a horizontal axis of travel?

Weight is always a force, dimensions MLT–2...

Weight is a force by definition. Its SI unit is newton. In most circumstances, weight is equal to $F=mg$.
The quantity in kilograms is called mass. In colloquial speech it is often called weight because we measure mass using the weight it exerts on a scale. In a technical context however, weight is always a force.
To make matters more complicated, there are units called "kgf" and "lbf"; kilograms-force and pounds-force respectively. One kgf is equal to the weight (force!) of one kilogram in Earth gravity.

Fortunately, the 'kilogram-force' or 'kilopond' is forgotten and no longer used. Its use, together with the 'real kg', of mass, has caused untold mistakes, and not only by students... It could be used more safely if mass was accounted in 'Technical Mass Units' or TME... But it was all a mess...

@xxavier If only the kgf was truly forgotten. It remains very appealing when talking about thrust or lift to many. I've lowered my expectations and as long as they use 'kgf' and not 'kg' when talking about forces (and refrain from using freedom units) I'm happy.

But for the formula in question they are talking about thrust being equal to Kilogram-Force as measured on a standard body scale for instance... yes?

Isn't force supposed to be measured in newtons, and mass in kilograms?

@nobody_loves_you Exactly. So 50kgf is about 490N.

@nobody_loves_you And the rules for which numerical value to use: the same rules apply as always when using mixed units. Ideally you convert everything to SI units before calculation, or your result will simply be in some nonsensical unit like foot-pounds per acre-ounce.

In vertical autorotation, with a constant sink velocity, the lifting force produced by the rotor is exactly equal to the weight of the gyro, since any difference between weight and lift would cause an acceleration, and we're considering a constant sink velocity, with zero acceleration.
In a dive under autorotation, with a stable dive path, the force produced by the rotor is higher than the weight of the gyro, since the tip-path plane of the rotor is tilted back, so you have a rotor drag component that has to be added (vectorially) to the vertical component (equal to the weight) in order to calculate the total rotor force.

One of these days the fixed wing folks will get what you are talking about.  They think lift is less than weight in a climb.  (Taking the engine away, and saying "now try it", can be helpful). Vbg and Va are faster than Vmin sink for reasons listed above.

@RobertDiGiovanni Rather than perpetuating your viewpoint in comments, bypassing SE's quality control measures (the infamous Downvote...), perhaps it is more constructive to post your point of view on the appropriate question concerning fixed-wing flight. I'm quite eager (if not very sceptical) to see your arguments why you think lift would not be less than weight in a climb.

What I'm trying to get a hold on is for the formula: $$ T = 2v^2· A·  rho $$ do I use the 'weight force' as the value for thrust? or do I calculate the thrust in newtons with: $$ T = F = m * a $$?

Thrust is a force, and the SI unit for force is the newton. If you multiply velocity squared times area times density, and you use m/s, m2 and kg/m3, you obtain the force in newtons. Automatically... Advantages of the SI system...

You should learn to use simple dimensional analysis. You can find it explained in any elementary physics book...

@Sanchises because the vertical lift is cosine lift, there for lift is greater than weight.  Don't be fooled by "vertical thrust component".  Thrust maintains airspeed, wing vertical lift renders plane "weightless".  It works best that way.  With the rotor, vertical drag spins the rotor, with the glider, vertical drag pushes it forward.  Once moving, the wing lift keeps it steady state.  Drag producing thrust, ha!  (A windmill produces electricity, or could be a propeller with an electrical source).

@Robert I agree that "thrust" is a poor choice of words for the force of a autogyro rotor - but then so is drag. Call it lift. (1/2)

@Robert Vertical force balance is $W=L\cos(\alpha)+T\sin(\alpha)$. Horizontal balance is $L\sin(\alpha)=T\cos(\alpha)$ (with $W$ weight, $L$ lift, $T$ net thrust, so thrust minus drag). Rewriting we get $$L=W\dfrac{1}{\cos(\alpha)+\dfrac{\sin^2(\alpha)}{\cos(\alpha)}} = W\cos(\alpha)$$ But once more, please post your point of view on the question linked.


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Wing Loading in a vertical loop
I have been sent some old intermediate Royal Navy exam papers from 1956, I took a similar exam in 1966. One question concerns Wing Loading in a vertical loop, i,e, at the top and bottom. I know the weight, radius, speed (remains constant) and wing area. Units are British Imperial. In the deep recesses of my mind I remember that:
W=L= Clrv^s where r is density in straight and level flight.
Question
what other factors or formulae do I need produce an answer.

You may add a link to your exam paper. Note that you can also use write math formula in a clearer way.

Is your question "how to compute the wing load at the top and bottom of a vertical loop?"?

In a real aircraft, speed won't remain constant in a loop without a LOT of throttle & speedbrake inputs, which aren't typically (probably, ever) accomplished. G-loading will be related to the radius of the loop, so even assuming a constant speed (works well in Physics class), you'll need that term in there. Of course, a constant pull will give you x+1 G's at the bottom of the loop, but x-1 at the top.

Wing loading will stay constant or slowly decline as you burn off fuel, but load factor will vary throughout the loop. Depending on how it is flown, it can be negative at the top and 3 or 4 at the bottom when 1 ist the value for straight flight.

@Manu H:  "You may add a link to your exam paper."  It's paper.  How do you hyperlink to paper?!  ;)

@MichaelHall by scanning it, searching a copy somewhere,... Old exams can be published on examination records.

Manu H yes looking for the forula.

@PeterKämpf The wing loading should not stay constant in the loop due to the following. The wing loading is defined as the mass of an object divided by the area of the wings (assuming the weight is equal to the lift generated by the wings). At the bottom of the loop, the "apparent" mass would be increased due to the sum of the weight and centrifugal force. At the top of the loop, those two forces would have opposite directions, which means lower "apparent" mass and lower wing loading than at the bottom of the loop.


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Why aren't 3-surface aircraft more popular in general aviation?
From everything I can gather, canards offer significant gains in performance characteristics, and most the drawbacks apply only if you remove the tail. Why not mix them together? What does Piaggio know that we don't?

With all the effort over the years to go with flying wings so there is only one surface, going for 3 is kinda going the wrong way doncha think?  Canards have historically been a flop, because of the compromises that come with the potential performance gain.  The Piaggio's speed largely comes from its laminar flow fuselage profile, not so much from having 3 surfaces.

I hadn't heard anything of such efforts. Were there any proposed GE flying wings? Are they a significant improvement over today's wing-and-tail design? Thay look like they will be a pain to get usable space out of.
What compromises come from having 3 surfaces?

In theory, the fewer surfaces the better, but flying wings have their own limitations, mainly a tendency toward twitchy pitch behaviour, and a very narrow center of gravity range, being so short coupled.  The single wing with a tail surface is nearly universal because it just meets the balance of compromises the best.

It's almost a flying wing, just chopped up and placed where it's needed most :P
My main contender is the Katmai, which is enhanced by the canard addition (which demonstrates another advantage, retrofitting rather than overhauling)

The original motive for a flying wing was to make the fuselage a useful lifting surface rather than a simple parasite, really nothing to do with reducing the number of surfaces. The low radar signal was discovered as a side benefit and so efforts moved in optimizing that aspect. 3 surface aircraft are mechanically redundant in the wrong ways, and really they just lack a market.

The first aeroplane ever to fly under its own power, the Wright Flyer, was a canard design. Once they better understood the issues of stability and control, the Wrights experimented with a three-surface arrangement before abandoning the foreplane altogether after the French pattern. Admittedly nobody yet quite knew what they were doing, but the Wrights' abandonment is symptomatic of the problems the canard surface faces.
As has been pointed out, the Piaggio Avanti adopted a foreplane primarily to shift the main spar aft. Another solution to that problem is to sweep the main wing forward, as was done on the HFB Hansajet. Both types are/were medium-size bizjets with the same problem - how to max the cabin size when a main spar keeps getting in the way. Forward sweep has been adopted by sailplanes on occasion for much the same reason, but a three-surface design would impair the airflow over the main wing.
Another successful three-surface family comprises the several variants and derivatives of the Sukhoi Su-27 jet fighter which added a canard, such as the Su-30 and Su-33, but here the goal is supermanoeuvrability.
I am less jaundiced over canards than some who have posted answers here - some designs have been outstandingly successful, although the last one in general aviation that I can recall off hand was the pioneer-era ASL Valkyrie. But I would say that they are a lot harder to get right than a tail, and only worth pursuing in exceptional circumstances - whether you keep the tail or not.

Canards are not used in general aviation simply because they have complexe interaction with the main wing which can leeds to issues with stability and being at the front of the CG their effect on flight dynamics may be highly unintuitive in some situations. Therefore you need electronic control to adapt the pilot input to the control surface in order for the plane to fly as expected, and that's why in my opinion it was not suited to GA where electronic was expensive until the past few years. But they are plenty of jet fighter using them as they have the ability to greatly increase performance and maneuvrability.
In addition 3 surface aircraft means more wetted aera thus more drag, complex design and control, more mechanical moving parts which will often drive the price up.
You can find more information looking at the patent issued in 1988 by Piaggio here : https://worldwide.espacenet.com/publicationDetails/description?CC=US&NR=4746081A&KC=A&FT=D&ND=&date=19880524&DB=&locale=en_EP

Just ask the MiG design bureau pilots who liked the MiG-8 Utka extremely well because of its easy flight handling. That was well before the time of electronic stability augmentation. Done right, a canard is a pleasure to handle. Only its performance will be poor.

I have seen static canards stapled onto existing aircraft, like the katmai, so I don't see how it would have to add more moving parts. You can also decrease drag because you no longer need tail down force

canards offer significant gains in performance characteristics

That is news to me. So far, I have viewed the canard wing as akin to an extended spoiler which messes up the flow over the main wing.
The benefit of a canard wing on the Avanti is to shift the main wing back so the main spar does not run through the cabin. Note that the canard does not have flaps, so it avoids some of the drawbacks of canard wings. Also, its span is small and its free vortices hit the engine gondolas, so they have no chance to mess up the spanwise angle of attack distribution over the wing (as most other canards do).
For pitch control and for trimming the airplane with flaps down, the Avanti has a regular T-tail, so the canard's task really is restricted to shifting the wing back. This is a clever design but its use is restricted to small business airplanes with straight wings. Airliners and most swept wing airplanes in general would not benefit from the small backward shift a similar canard wing could offer, so their conventional layout has merits. In most GA aircraft, the spar runs below the pilot seat and can well be integrated with the general layout, so again an added canard wing would not improve the design.

"Restricted to small planes with straight wings". Or presumably with forward-swept wings

@MSalters: Right, but even more exotic. The HFB-320 Hansa Jet comes to mind. Sweep was generous enough to not even require that additional canard wing.

The reason I think there's a gain to be had is potentially eliminating tail down force, and while I was researching it, found an aviator quoted as saying "I fly a canard equipped Cessna 182 and it improves all aspects of the performance envelope". So my suggestion of performance gains is partially theoretical, partially tested and approved

@JasonSummer: These canards help for short take-offs but hardly improve cruise fuel flow or top speed. This quote is from someone who has something to sell.


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Where does the air enter the passenger cabin?
After passing through the packs and turbines, where does the air get into the cabin? On the 737-800, for example. I don't see on the roof any grill to let the air enter the cabin, look:

Source: Airliners.net
Where does the air enter the cabin? According this graphic, the air enters through the roof and exits through the wall:

check the floor, that's where the packs are.

@ratchetfreak You mean the grills on the wall? I asked a question about those and their purpose is cabin air circulation

"Circulation", ergo where cabin air enters and/or exits

@kepler22b They said it was for pressure balancing between the cargo compartment and the passenger compartment.
There are those ceiling nozzles above the seat that the air comes in at the least.

@ratchetfreak I'll put a picture to show that the air enters the cabin on the roof and exits through the grills on the wall :)

@SMSvonderTann Yes. The one who answered my question said that it was for cabin air circulation, though he didn't have any technical reference to say that the grills also provided pressure balancing

I see a big opening in the ceiling, that's were the air comes from. See here and here. Here a nice 3D view of the ducts, but is focuses on the cockpit.

It looks to me in your top image that the air vent is right there at the center of the ceiling.

It looks like a linear slot diffuser running the length of the cabin.  Similar to this


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Why are these rivets on the 777 so visible?
Look at this 777 nose:

Source: Airliners.net
It is full of rivets.Why are they so visible on this particular aircraft? Why doesn't the radome have rivets?

Looks like she's had a fresh coat of paint since then.

The radome needs to be transparent to electromagnetic radiation, so it cannot be metallic, but is a composite shell. Bonding replaces riveting here.

At the risk of sounding ridiculous, the purpose of the rivets is to hold the plane together. More specifically, they hold the outer aluminum skin to the structural skeleton of the aircraft:

(That skeleton happens to be made of manilla folders, but the real one is pretty much identical except that it's bigger and made of metal.)
There are also additional rivets around things like the AOA vane, holding doublers in place to stiffen the skin where these instruments penetrate and create additional load.
The radome has no rivets as it is a single molded piece of plastic. It does however have structural stiffeners, which you can see running fore-to-aft in the photo.
You can see the rivets clearly on this particular aircraft because of the paint scheme (all white) and lighting, however if you look closely you can spot them on any 777 nose.
Here's one from an aircraft during assembly, and as the interior has not been installed you can also see the skeleton through the windscreen openings:

I would also hazard to guess that this particular aircraft's paint job is nearing the end of its useful life and the rivets are showing paint wear.

@JonathanWalters is probably right - these are all flush rivets. It's either bad paint, or weird lighting, or an HDR photo, or some combination.


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Why are some aircraft windows painted over?
I have seen that some aircraft have some of their windows painted white. As you can see in this picture, this A340 has many windows painted white.
Why is it that?

Source

Same question as this: http://aviation.stackexchange.com/questions/23188/why-would-a-passenger-airliner-be-missing-windows-in-the-fuselage/23189#23189

If you look at the seating chart for that plane, you'll notice that those windows match the location of toilets and galleys.
Note that the exact position of toilets and galleys may change during the lifetime of an aircraft, so it's easier to have all the windows installed by design, and just obstruct some that are not used. Even if it were just obstructed from within, I suppose it's better if in an emergency, if emergency personnel had to look inside the plane (to check for people still trapped inside), they would be able to identify quickly those windows that do not need looking through.

It means that there is some sort of internal structuer (like a wall or a cooking unit) behind that window on that particular configuration of the aircraft.  Manufacturers are required to certify the planes with windows in place in order to show that they don't suffer from metal fatigue after repeated flights.  So moving a window (or covering it up with metal) is a lot harder than just putting a little plastic cover on the inside of the window.  Hence, when a particular configuration places a wall or a cooking unit there, they just cover it up.

As I understand the statement, the aircraft is certified with its maximum number of windows. Changing the window configuration changes the structure of the air frame, requiring a new certification, therefore, the windows are simply covered from the inside, leaving the air frame unmodified and the original certification still valid.

That is tested for as part of the certification process. Modifying the frame to plug the window (instead of just covering it) would change the way stress propagates through the metal, and would change the rate of fatigue, thus needing a new round of certification. Covering or painting over the window is much simpler and cheaper.

The story of the De Havilland Comet, https://en.wikipedia.org/wiki/De_Havilland_Comet. shows why "windows" and "metal fatigue" are closely connected...

@DJohnM Bingo, I thought about including it but figured people would likely look up metal fatigue on their own.  But yes, that is the perfect example of why it matters.


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Why is air mixed with bypass air on the A/C of an aircraft?
Air is taken from the compressors, then it is cooled by the air that comes from the ram air inlet. After that, it is compressed and cooled again by an expansion turbine. When finally air is condensed, it mixes with bypass bleed air. What is bypass bleed air?

You almost have the answer in your question; bleed air is hot, compressed air taken from the engine compressors.  Bypass bleed air here refers to some of that air which is bled off the engine, but not routed through the air cycle machine (pack).  When the air comes out of the pack, its really cold.  The pilot has a knob which blends the cold air from the pack with the hot bypass air to adjust the temperature that comes through the actual ductwork; this is similar to the blender box in a car.

For example, in this image taken from this answer on SE Aviation, the bleed air from the engine comes in at the top right corner, and you can see the line for bypass bleed air along the top of the diagram in the pipe labelled "hot air".

In short
Bypass bleed air is a fraction of the bleed air coming from the core compressor. It's named like this because it doesn't cross the air cycle machine of the packs as does the regular bleed air.
On the following picture this bypass bleed air is identified as trim air. Its purpose is to be mixed with cooler bleed air from the packs to modulate the temperature in the different zones.
Mixing is done by the mixing and distribution unit (see more below).

Use of trim air on the Airbus A320 family. (Source: A319/A320/A321 ECS Familiarization and Training Manual)
Boeing has open a new approach for air conditioning and pressurization using ram air rather than bleed air. The first aircraft with this approach is the B787, this requires dedicated compressors.

Details
The air conditioning and pressurization systems on large airliners are quite similar. Here is the overall picture:

Air conditioning and a part of the pressurization are obtained using the environmental control system (ECS)
Bleed air, the high pressure air taken from the engine cores and APU, is very hot.
Ram air, the cool low pressure air taken from the aircraft belly, is used to cool the bleed air in heat exchangers. This ram air, which gets warmer in the process, is rejected in the atmosphere without entering the cabin. A fan is used to create the ram air flow when the aircraft is on the ground.
Air conditioning packs are mainly air cycle machines (refreshers) that receive pre-cooled bleed air and deliver air at ambient temperature. There are usually 2 or 3 packs for redundancy.Some bleed air bypasses the ACM and can be used later by the mixing unit.
Air from the packs goes to the mixing and distribution unit which role is to mix air from packs with air already in the cabin, and deliver the result to different zones of the aircraft. The unit can vary temperature according to zones by mixing packs output air and bypass bleed air. The unit also mixes air from the ground supply when at the gate and engines and APU are not running.
Air in the cabin is recirculated by two or more fans which return it to the mixing unit.
As air is pumped into the aircraft by the mixing unit, air also exits the aircraft through one or several outflow valve(s). The exact quantity of air leaving the aircraft is controlled so that the pressure inside the aircraft is at the required value.This value is usually expressed as cabin altitude, that is the altitude we would find this pressure in the standard atmosphere. Pressure is about 1013 hPa at sea level and then decreases continuously when altitude increases.However the cabin altitude doesn't climb as high as the aircraft because there would be an insufficient oxygen pressure in the cabin. The cabin altitude is kept proportional to the aircraft altitude, but usually the minimum of pressure is a value equivalent to 6,000 to 6,500 ft.
All those elements are driven by controllers that can be monitored and programmed by the crew.

Let's look at the ECS for the Airbus A320.

1. Location of elements

(When not specified, the source of the images is the A319/A320/A321 ECS Familiarization and Training Manual)

2. Packs
A view of a pack with the triangular ram air inlet (the ram air outlet has been removed):

The pack includes:

Two air-to-air heat exchangers: main (MHX) and primary (PHX). Ram air is used as the heat receiver fluid for both.
A centrifugal compressor (COMP) and a turbine (TURB). They compose the air cycle machine (ACM) which cools and reduces pressure of bleed air.
A reheater (RH), a condenser (COND) and a water extractor (WE) to dry bleed air.

3. Air cycle machine

4. Pack operation
A pack works this way:

Hot high pressure bleed air is directed to the PHX which cools it a bit so that it can be processed more efficiently by the ACM.
Cooled air is sent to the compressor. Increasing the pressure allows for more heat to be extracted in the next step. Air gets very hot again.
Compressed hot air is sent to the MHX which remove as much heat as possible, possibly below the ram air temperature, which is great while on the ground.
Cool air, which contains water vapor, goes to a reheater and a condenser to create liquid water, water is collected by the water separator. This water is reinjected into the ram air to cool it before it is used in the exchangers.
Cool air finally rotates the turbine which extracts energy from it to drive the compressor and a ram air fan. The energy loss produces a cool low pressure air which can be used in the cabin and cockpit.

The exact temperature and moisture of air delivered by the pack are obtained by mixing air from the ACM turbine, cooler air from the PHX and warmest bypass bleed air taken upstream the MHX.
Water extraction is controlled by adjusting the temperature, in the same way, of the airflow going through the condenser.
Bypass bleed air is also delivered by the pack to the mixing unit for zone temperature trimming.

5. Water extractor

Collected water is reinjected into ram air flow.

6. Air temperature and pressure at each step

FCV: Flow control valve (bleed air input)

There are 4 phases to an internal combustion engine, even a gas turbine: suck, squeeze, bang, blow.

Suck: Air is sucked into the engine
Squeeze: The air is compressed by the turbine
Bang: The concentrated air is mixed with fuel and ignited.
Blow: The expanded gases from the ignited fuel push out the back of the jet

Bleed air is taken out of the engine after squeeze but before bang

The Environmental Control System (ECS) which supplies air to the cabin usally uses bleed air from the engines in most aircraft. The figure below shows the engine bleed air system in a Boeing 767.

Image from Commercial Airliner Environmental Control System- Engineering Aspects of Cabin Air Quality by Elwood H. Hunt et. al, from Boeing
The main air supply for the ECS usually comes from the compressor, where air is taken from high/low pressure areas. These are used during different phases of flight.
The air is sometimes taken from the bypass air of the engines for the purposes of cooling. As the air is compressed in the compressor, the temperature of the bleed air rises (especially the one from HP stage) and the bypass air is used to cool it.

These answers are all correct, however. It should be noted that there are 2 separate types of bypass bleed air involved in this air conditioning system.
 There is bypass air taken just downstream of the flow control valve at the entry to the pack system that goes straight to the trim air for cabin temp control.
 The other bypass bleed air is involved with preventing ice buildup in the condenser/water extractor. this is the air that flows through the bypass valve(BPV), bypassing the compressor and main heat exchanger to provide hot air downstream of the turbine. The valve opens based on difference in pressure due.

We recently had a NGS problem on a B-737. The fault was the air was too cold entering the system. Troubleshooting was accomplished and the pre-cooler valve on the engine supplying bleed air was stuck open. My question is, what effect, if any, would show up with the air conditioning system the bleed air is supplying>

Hi Mitchell,  Welcome to Av.SE!  That's a good question, and you should ask it as a NEW question.  In the format here, "question and answers," posts made as "answers" are expected to, well, provide an answer to the stated question.  So asking a follow-up question in this thread won't work well.  But this would be a good question on its own.

authors: سامسونگ چینی


text:
When do commercial flights turn off the APU?
On commercial flights do the pilots ever turn off the APU? Is it needed when flying?

This varies due to operational need and airline standard operating procedures.
On my aircraft (an Embraer E-Jet) we turn on the APU roughly ten minutes prior to pushback and shut it down shortly after starting the engines.
If we plan on taxiing on one engine we will shut it down after the first engine start and perform a cross-bleed start on the second engine. Other operators leave the APU running for second engine start.
If performance requirements demand the engine bleed to be off for takeoff we will leave the APU on until after takeoff to provide cabin pressurization until the engine bleeds come back online.
We typically leave the APU off for the remainder of the flight and do not turn it back on until the next flight, unless ground power is unavailable or unreliable at the destination, in which case we will turn it on after landing as we approach the gate.
The APU can also be left on (or started in flight) to assist with various abnormal or emergency situations where bleed air or electrical power generation might be needed. Each APU installation will have a different flight envelope in which it can be started / used and is specified in the aircraft's operational limitations.


text:
Why is the front part of the engine not painted?
The engines are always painted a certain colour but the front part is always grey.
Source

And here on the XWB:

Source
Why is it that?

Not directly related to your question but what's the red line on the side of both engines that sort of goes halfway around?

Red line is to indicate danger area. No personnel allowed in front of red line when engine is running. http://www.boeing.com/commercial/aeromagazine/articles/qtr_3_08/article_04_3.html

The "no people" symbol suggests that the red line indicates how far back from the inlet you should remain for safety.

@curious_cat if you are standing forward of that line and the engine is spooled, you are going on a fatal trip through the N1 fan.

@casey like This?

@TomMcW that guy got lucky. I know some flight attendants that watched a mx tech turn into a pile of goo and chunks no heavier than 2 lbs during a static runup on a 737.

@casey Ouch! Yes, that guy got super lucky. Whoever manufactured that helmet he was wearing should be commended. It had to have busted every compressor blade for him to have survived that. 12 stages in the J52 on the A6

@TomMcW According to the narration, a strap on his suit snagged on a probe and held him in position a few inches in front of the fan. The engines themselves are several feet aft of the front of the intake.

@reirab  Ah. I didn't watch the whole thing. I've seen that video a thousand times. It's so freakish you can't stop watching. But I wouldn't want to see what the flight attendants saw that Casey mentioned. I guess the comp blades must be pretty far back from the inlet. He wouldn't get so lucky on a high bypass engine. The fan is right there. Like a giant Cuisinart!

@vasin1987 - I especially like how that article is entitled "Preventing engine ingestion injuries...." as if "being turned into mince" still falls under the "injury" banner

@casey - Preposterous! What's the worst that could happen?

The engine inlet lip is bare metal to facilitate anti-icing. The area is heated from within using hot engine bleed air.
This partial schematic (from Boeing AERO QTR_01.12) shows the engine anti-ice valve, which controls the air supply to the inlet lip:

Interesting side note: Even the 787 "no-bleed" system architecture uses bleed air for engine anti-ice.

There are two reasons I can think of for this:

Aerodynamic reasons. Believe it or not, even paint can change the dynamics of a plane, to the point that Boeing discourages  multi-colored paint jobs or liveries on 787 nacelle, claiming they affect performance.
If that part of the engine were painted, there would be risk of paint chips being sucked into the engines, potentially damaging them. While this may seem far-fetched, this account shows that even paint chips can pose a foreign object debris threat to an engine. By not painting this part of the nacelle, the risk of paint flecking off and into the engine is reduced.

The linked article is about chips of concrete and runway marking paint causing damage. The stuff they use to paint the runway markings is much more substantial than the paint used on planes.

what is the second reason ?


text:
If there was an engine malfunction, could we breathe contaminated air?
If the engine on airliners has for example, an oil leak, can the oil or other substances contaminate the air on the passenger cabin? How would this situation be avoided?

Traditionally, an engine failure means it stops spinning... not that it is working incorrectly.  I believe the term for what you're describing is usually a malfunction.

Just watched an NTSB program on tv the other day where a Cessna had an incorrectly installed exhaust system. The cockpit filled with carbon monoxide. The pilot became incapacitated and crashed

Or read this carbon monoxide incident - http://www.avweb.com/news/news/182437-1.html

So both of those examples are for smaller GA planes... is this question about those types, or airliners?

It can happen on airliners as well, yes.

@fooot The question is about airliners

Yes, it is possible for contaminated air to enter the cabin. The air pressurization is provided by the "packs" (air conditioners), which are in turn powered by the bleed air (hot air extracted from low stages of the turbine) of the engine:

To avoid the problem, bleed air is shut when engine trouble is confirmed or suspected. For example, the Engine Fire checklist for the Boeing 737 calls for:

[Autothrottle]...Disengage
[Thrust lever]...Confirm...Close
[Engine start lever]...Confirm...CUTOFF
...
[ISOLATION VALVE]...CLOSE
[PACK switch]...OFF

With one engine out, the cabin air is provided by the bleed air of the other good engine. With a dual bleed shutoff (or failure), you better get down right away.

Yes, an malfunction in a jet engine can cause contaminated air to reach the passenger cabin. It is a rare occurrence, but it does happen.
One of the most famous cases was British Midland Flight 92 on Jan 8th 1989 in Kegworth England.
Kegworth air disaster
"After taking off from Heathrow at 7:52 p.m., Flight BD 092 was climbing through 28,300 feet to reach its cruising altitude of 35,000 feet when a blade detached from the fan of the port (left) CFM International CFM56 engine. While the pilots did not know the source of the problem, a pounding noise was suddenly heard, accompanied by severe vibrations. In addition, smoke poured into the cabin through the ventilation system and a burning smell entered the plane".
In addition to contamination from engine failures, there is a much more public debate over contamination due to perceived weaknesses in the design of the aircraft cabin air systems. These weaknesses have not been positively identified but many believe it is due to faulty engine seals and/or air conditioning packs. There are numerous reports of pilots, flight attendants, and passengers feeling ill from "toxic air" on aircraft.
The name "Aerotoxic Syndrome" is also used to describe the problem of bad air in aircraft. More information here: Aerotoxic syndrome
Here is a website dedicated to Toxic Cabin Air. Toxic Cabin Air

Just an aside.  The crew had recently transitioned to the 734 from previous "steam gauge" versions.  On the aircraft they were familiar with, bleed air for the cabin was only taken from no 2 so the presence of smoke in  the cabin added to their mistaken diagnosis that the problem was with that engine although the 734 takes cabin bleed from both engines. The second problem was that the vibration indication moved in the opposite sense to the steam gauge which reinforced the diagnosis and led to shutting down no 2. As usual, a lot of holes in the cheese lined up in that disaster.

Jet engines on airliners take some relatively low pressure bleed air from the front stages of the compressor in order to feed compressed fresh air to the cabin and to power other items.  This air is as fresh as the outdoor air.
Note that when a commercial airline starts engines on the ground at an airport with a significant tailwind then the engine can ingest some of its own exhaust and therefore upon occasion as a passenger you can smell the odor of kerosene.
If an engine is shut down in the air for some reason then no pressure is generated and other engines on the plane will supply the compressed air to the cabin.

It happens reasonably frequently.

The problem is more about lubricant or hydraulic fluid leaking and reaching high temperatures and then decomposed into toxic or carcinogenic elements.

@Mark Thanks.  I edited my answer to remove the last paragraph.